Make the default mixer command a compile-time option and rename mixer
name to mixer command since it is interpreted a command which can be
optionally specified with full path and command line options.

Use xfce4-panel debuging methods described on
http://docs.xfce.org/xfce/xfce4-panel/debugging#debugging_plugins.
In particular log to stderr and enable debug logging if the PANEL_DEBUG
environment variable is set to "all".
Change G_LOG_DOMAIN to the more intuitive pulseaudio-plugin.
Remove logging to a logfile as multiple plugin instances clobber the
logfile and automatic restart after abort() or crash truncates the
file.
Use pulseaudio_plugin_debug instead of g_debug which provides
additional information.

Conflicts:
	panel-plugin/pulseaudio-plugin.c

(Harald Judt)

Wired up a "mixer-name" property.

Commented out lines demonstrate the issue with libintl setup:
- compilation warning (implicit _() definition)
- runtime crash (no '_' symbol)

Forgot to add them in the previous commit.

enable-keyboard-shortcuts property not yet working (?)

Icons audio-volume-...-panel used in sound-indicator are one size only.

Button with an icon reacting to volume changes and to mouse wheel events.